Fluid transfer devices are known that operate in a first direction as a pump and in a second direction as a motor. These devices may comprise a housing within which a rotor rotates with respect to a port plate and a cam plate angled with respect to the rotor's axis of rotation. The rotor includes one or more bores (generally an odd number) each for receiving a piston. One end of each piston held in contact with the cam plate. As the rotor rotates with respect to the housing, each piston moves axially with respect to the rotor and the port plate.
The port plate includes a fluid inlet through which a fluid enters the housing when a piston aligned with the fluid inlet moves away from the port plate and a fluid outlet through which fluid exits the housing when a piston aligned with the fluid outlet moves toward the port plate. When the rotor is connected to a source of motive power, the rotation of the rotor causes the pistons to draw fluid from the inlet and expel fluid through the outlet; when operated in this manner, the fluid transfer device is referred to as an axial piston pump. When fluid is applied under pressure to the fluid inlet and drawn from the fluid outlet at a lower pressure, the rotor is caused to turn by the pressure difference; when operated in this manner, the fluid transfer device is referred to as a hydraulic motor. Thus “axial piston pump” and “hydraulic motor” may refer to the same fluid transfer device, depending on the what is making the rotor turn. Such devices are disclosed, for example, in U.S. Pat. No. 5,809,863 to Tominaga and in U.S. Pat. No. 5,850,775 to Stiefel, the disclosures of which are hereby incorporated by reference.
Friction develops between the moving pistons and the rotor cylinders in which they are housed. Therefore, it is known to form the pistons of a wear resistant tool steel. One suitable tool steel that has been used with satisfactory results is a vanadium containing material available from the Crucible Materials Corporation of Syracuse, N.Y. under the designation CPM 10V. In use, a piston formed entirely of CPM 10V is heat treated and then surface hardened using a nitriding process to increase the piston's wear resistance. Such pistons perform satisfactorily in many environments. For various reasons, including improved machinability, however, the sulfur content of CPM 10V has recently been increased from about 0.07 percent to about 0.14 percent. It has been found that this higher level of sulfur adversely affects the fatigue strength of pistons formed from this material. For many applications, it is not commercially practicable to obtain an alloy equivalent to the old formulation of CPM 10V.